Depilation using dichromate and strong acid



United States Patent 3,482,924 DEPILATION USING DICHROMATE AND STRONG ACID Francois Bernard Marie Grall, Chantilly, and Michel Pierre Yvon Dalle, Creil, France, assignors to Ugme Knhlmann, Paris, France No Drawing. Filed June 7, 1967, Ser. No. 644,053

Claims priority, application France, June 8, 1966,

Int. (:1. (3146 1/06 US. Cl. 8-94.16 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for the depilation of a skin which comprises treating it in a bath containing at least 10 g. per litre of an alkali metal dichromate and a strong mineral acid and then if desired subjecting it to an alkaline treatment.

The present invention relates to a process for the depilation of skins intended for the manufacture of leather. A process for the depilation of skins has recently been proposed in which chlorine dioxide in acid medium is used. The dissolution of the hair is in this case due to the rupture of the disulphide bridges and to the formation of sulphonic groups along the protein portions.

The present invention relates to a new oxidising process of depilation which comprises treating the skins in baths containing at least 10 g. per litre of an alkali metal dichromate and at least one strong mineral acid, and then if desired subjecting them to an alkaline treatment. The process according to the invention is based on the oxidation of the hair by the H+/Cr O couple. The hair then goes through the following states of oxidation Ker-S-S-Ker 1 KerSO ---SKer (2 KerSO SO Ker (3 KerSOH (4) KerSO H (5 KerSO H (initial formula of the hair, Ker signifying keratin) If the oxidation is carried sufiiciently far, the states (4), (5) and (6) are obtained, where the hair dissolves through rupture of the disulphide bonds.

If the oxidation is not carried sufiiciently far, the forms (1), (2) and (3) preponderate and the skin keeps its hair. Depilation can then be obtained by hydrolysis of the oxidised derivatives by means of an alkaline treatment, for example with a concentrated alkali:

Sodium and potassium dichromate may equally be used for carrying out the process of the invention. The strong mineral acid may be, for example hydrochloric acid, nitric acid, sulphuric acid, or perchloric acid. Either one acid alone or a mixture of acids may be used. The acid content of the oxidising bath is preferably greater than 6 g. per litre. It is advisable to dis- 3,482,924 Patented Dec. 9, 1969 ice solve the dichromate in water, then add the acid and finally the skins. The oxidising treatment may be carried out directly on the cleaned, softened and fleshed skins. In order to obtain the defibrillation and flexibility usually produced by depilation with lime, it is enough to regulate the ratio H+/Cr O and also the contact time and the temperature of the bath. The nature of the acid used has a certain amount of influence on the oxidation of the disulphide bridges. It would appear that, besides the value of the oxidation-reduction potential of the mixture, there may be a specific action of the anions. In spite' of the strong acidity of the baths, appreciable swelling of the skins is not found and the addition of neutral salt is not essential. The apparatus used-vats, stirring apparatus, fulling mills-must resist the corrosive action of the H+/CI2O7 couple.

When the oxidising bath is not sufliciently concentrated to break the disulphide bonds of the keratin, the depilation can then be accomplished by following the dichromate treatment with an alkaline treatment. In this case, however, it is advantageous to protect the hair side before the alkaline hydrolysis by a treatment with quinone, formaldehyde or other tanning agent. One may also resort to rinsing for this purpose.

The depilation process according to the invention enables chrome tanning to be carried out in-particularly advantageous conditions. The conventional process of preparation of leathers for chrome tanning comprises depilation effected generally with a mixture of lime and sodium sulphide or hydrosulphide. The skins are then brought back to a pH near to neutrality by treatment with acids, sulphites, bisulphites or ammoniacal salts. For the chrome tanning, two processes may be used: the process known as the one bath process, in which the delimed skins are acidified by treatment with solutions of mineral or organic acids in the presence of salt, then treated with salts of trivalent chromium, the basicity of which is progressively increased; the process known as the two-bath process, in which the skins are first treated in a bath containing sodium dichromate, an acid and frequently a little salt. The chromic acid thus fixed on the skins is then reduced in a second bath consisting of a mixture of sodium thios-ulphite and hydrochloric acid.

When the skins have been subjected to the oxidizing treatment according to the invention, which may be total or partial, chromic acid is fixed on their fibres. This acid may be reduced in situ by the two-bath tanning technique. The depilated chrome-tanned skins then follow the course of the usual subsequent operations. Skins which have undergone the oxidising treatment, then the reducing treatment, but are not yet depilated, are subjected to alkaline hydrolysis under the conditions recommended above. After partial oxidation, the not yet depilated skins may also be subjected directly after rinsing to alkaline hydroylsis and, after depilation and pickling, undergo a tanning treatment by the one-bath process, the tanning liquor being prepared by reduction of the residual depilating bath, from which the sludge may have been removed.

In comparison with the process using chlorine dioxide in acid medium the depletion process according to the invention offers the advantage that the products which have served for the depletion process itself can be used for tan- 11mg.

It also enables the volume of polluted water to be reduced and the use of sulphides and hydrosulphides to be avoided, thus making the problemof purifying the waste water from the tannery less arduous. Finally, since the skins have not shown any swelling, the natural defects are less accentuated on the finished leathers.

The invention is illustrated by the following examples:

3 EXAMPLE 1 (a) 31 kg. of commercial sodium dichromate are dissolved with stirring in 250 litres of Water in a vat, and 31 kg. of 62% nitric acid are added with continuous stirring. A check is made that the temperature of the bath is not over 25 C. 100 kg. of skins hung on a stainless steel frame are immersed in the bath. The centre of the frame is connected to a continuous agitation system.

At the end of 24 hours, the hair forms a sludge dispersed in the bath. The skins are then placed in a fulling machine containing 500 litres of a 20% solution of sodium hyposulphite. They are fulled for a quarter of an hour, then left in the fulling machine for 24 hours with a quarter of an hours rotation in each hour. At the end of this treatment the shrinkage temperature of the leathers is 97 C. The leath is left to mature for 48 hours on a rack before it is neutralised, dyed and nourished as usual.

(b) The same method of operation as in (a), but using:

Water litres 250 62% nitric acid kg 28 Sodium dichromate kg 37 Skin kg 100 Depilation time hours 36 (c) The same method of operation as in (a), but using:

Water litres 250 62% nitric acid kg 25 Sodium dichromate kg 42.5 Skin kg 100 Depilation time hours 48 EXAMPLE 2 100 kg. of cleaned, softened and rinsed skins are fulled for 12 hours in a bath containing the following ingredients:

Water at 20 C litres 200 Crystalline sodium dichromate kg 6.8 62% nitric acid kg 20 The skins are left to stand overnight, and the next day they are rotated for an hour and the bath is emptied.

500 litres of a 20% solution of sodium hyposulphite are put in the fulling machine through the hollow axle, and it is rotated for minutes, and then for 15 minutes each hour for the next 24 hours. The leather is then treated for 12 hours in a bath comprising:

Water at C litres 400 Quinone kg 1.6

The leather is left in the fulling machine to stand overnight, and the next day is rotated for an hour. The pH is 6. Half the bath remaining from the preceding operation is evacuated, and 2 0 kg. of 40% commercial caustic soda are added through the axle of the fulling machine, the addition being made in a quarter of an hour. Rotation is continued for three quarters of an hour. The hair gelatinises and is dispersed in the bath. The latice-gate is then put in position, the cold-water inlet through the axis is opened and rotation is carried out for an hour. The gate is replaced by a solid gate and rotation is effected for an hour with the following bath:

Percent Water 300 Ammonium sulphate 2 A check is made that the leathers are at a pH near to 7, then they are rinsed. The leathers thus treated are then dyed and nourished as usual.

EXAMPLE 3 The skins are treated as in Example 2, but the retanning with quinone is replaced by retanning with 7% of 40% formaldehyde.

4 EXAMPLE 4 100 kg. of cleaned, softened and rinsed skins are fulled for 12 hours in a hath containing the following ingredients:

Water at 20 C. litres 200 Crystalline sodium dichromate kg 6.8 62% nitric acid kg 20 The skins are allowed to stand overnight. The next day rotation is effected for an hour and the bath is emptied. The spent bath is recovered. The skins are then rinsed twice in the fulling machine for two hours with 400 litres of brine containing 100 g. of salt per litre. The object of these two rinsings is to eliminate chromium compounds which, during the soda treatment, would have a tendency to make the hair side of the finished leather brittle. The chromium which is not eliminated is reduced by the dermis and fixed on the skin (beginning of tanning). After rinsing, fuiling is carried out for 10 minutes with 200 litres of a 40% caustic soda solution at 100 g. per litre. The hairs are completely dissolved, and are eliminated by simple rinsing for an hour, the solid gate being replaced by the lattice gate. The skins are then treated for an hour in a fresh bath containing the following ingredients:

Water at 20 C. litres 300 Ammonium sulphate kg 2 The skins, the surfaces of which have thus been neutralised, are then pickled and tanned by the usual process, the tanning liquor being prepared by reduction of the residual nitrochrornic bath possibly filtered, with the usual reducing agents.

EXAMPLE 5 In a falling machine resistant to the nitrochromic mixture, 19 kg. of commercial sodium dichromate are dissolved in 150 litres of water with stirring, and 26 kg. of 62% nitric acid are added, with continuous stirring. The temperature of the bath is checked so that it is not above 25 C. Then 100 kg. of cleaned, softened and fleshed skins are put in the fulling machine and are fulled for 12 hours, and left to stand overnight. The next day they are rotated for an hour so as to complete the dispersion of the residual hairs in the bath, then the skins are put in a falling machine containing 500 litres of a 20% solution of sodium hyposulphite. They are fulled for a quarter of an hour, then left in the falling machine for 24 hours and given a quarter of an hours rotation in each hour. At the end of this treatment, the shrinking temperature of the leathers is about 97 C. The leathers are left to mature for 48 hours on a rack before they are neutralised, dyed and nourished as usual.

The method of operation is as described in 5 (A), but with the following modifications:

Commercial sodium dichromate kg 19 Water litres 150 38% hydrochloric acid kg 39 Skin kg 100 Commercial sodium dichromate kg 19 62% nitric acid kg 16 38% hydrochloric acid kg 15.5 Water litres 150 Skin kg 100 Commercial sodium dichromate kg 19 62% nitric acid kg 12.5 sulphuric acid kg 7.7 Water "litres" 150 Skin kg Commercial sodium dichromate kg 19 65% perchloric acid kg 11.5 62% nitric acid kg 11 Water litres 150 Skin kg 100 Commercial sodium dichromate kg 19 65% perchloric acid kg 7.7 95% sulphuric acid kg 7.7 Water litres 150 Skin kg 100 Excellently tanned chrome leathers are thus obtained.

We claim:

1. Process for the depilation of a skin which comprises treating it at a temperature not above 25 C. in an aqueous bath containing at least 10 g. per litre of an alkali metal dichromate and at least 6 g. per litre of a strong mineral acid.

2. Process according to claim 1 wherein the dichromate is dissolved in water, then the acid added and finally the skin.

3. Process according to claim 1 wherein the acid is hydrochloric, nitric, sulphuric or perchloric acid.

4. Process according to claim 1 wherein the dichromate is sodium or potassium dichromate.

5. Process according to claim 1 wherein the treated skin is subjected to an alkaline treatment.

6. Process according to claim 5 wherein the alkaline treatment is effected in a bath of caustic alkali.

7. Process according to claim 5 wherein the hair side of the skin is protected by rinsing or by treatment with a tanning agent before the alkaline treatment.

8. Process according to claim 1 wherein the treated skins are subjected to chrome tanning.

References Cited UNITED STATES PATENTS 3,294,479 12/1966 Rosenbusch 8-94.16

GEORGE F. LESMES, Primary Examiner J. R. MILLER, Assistant Examiner US. Cl. X.R. 

